The object of the present invention is a lighting and/or signalling device equipping a motor vehicle, comprising at least one optical guide capable of producing a homogeneous diffusion of the light. This optical guide comprises prisms which make it possible to deviate the light rays.
The invention finds applications in the field of vehicles travelling on roads and, in particular, motor vehicles.
In the field of motor vehicle lighting and signalling, various types of device are known, amongst which there are found essentially: lighting devices situated at the front of the vehicle with, in particular, vehicle headlights equipped with dipped or low-beam headlights, having a range on the road close to 110 meters, and full-beam headlights having a long illumination range and producing an area of vision on the road close to 200 meters; lighting devices situated at the rear of the vehicle with, in particular, reversing lights; signalling devices situated at the front of the vehicle with, in particular, sidelights, direction indicators and D.R.L.s (Daytime Running Lights) (integrated or not with the headlights taking on the lighting functions mentioned above); and—signalling devices situated at the rear of the vehicle with, in particular, fog lights, rear lights, direction indicators and stop lights.
At present, use is known, in lighting devices or signalling devices, of one or more optical guides for propagating a light beam. An example of a vehicle headlight is described in the document U.S. Pat. No. 6,107,916. This headlight comprises a light source and an optical guide, placed in proximity to the light source and propagating the light beam emitted by this light source. This light guide can run along all or part of the glass or reflector of the headlight.
An example of a headlight is depicted in FIG. 1. More precisely, FIG. 1 depicts schematically the left-hand headlight of a vehicle. This headlight emits a light beam directed essentially towards the front of the vehicle, that is to say along the axis Y of the road. This headlight 1 comprises a protective glass 2 forming the output face of the headlight 1. It also comprises: a light source 3, emitting a light beam whose emission direction is depicted by an arrow 4; and an optical guide 5, propagating said light beam 4.
The optical guide 5 is a cylinder of transparent material provided with prisms, which provides the propagation of the light beam 4 from an end e1 close to the light source 3 to an end e2 opposite to the end e1. This optical guide 5 can have different geometrical shapes. It can, for example, form a circle, an arc of a circle or else be rectilinear. In the case of FIG. 1, the optical guide 5 follows the shape of the protective glass 2 of the headlight 1.
An example of the optical guide 5 of this headlight is depicted in more detail in the FIG. 2. This FIG. 2 shows a sectional view of the optical guide 5. This optical guide 5 comprises two faces: a first face 6 constituting an output face for the light rays propagated in the optical guide 5; this output face 6 is smooth and continuous; and—a second face 7, opposite to the first face 6 and constituting a reflection face of the optical guide; this reflection face 7 has a serrated profile, that is to say a profile in the shape of sawteeth. This reflection face 7 comprises a series of identical and symmetrical prisms 8. These prisms 8, placed side by side, form the sawteeth of the reflection face 7.
In FIG. 2, the optical guide 5 is depicted in a sectional view. For a better understanding of the figure, it is depicted hatched in FIG. 2. Thus, according to the sectional view of FIG. 2, each prism 8 has a substantially triangular shape. More precisely, each prism 8 has the shape of a triangle comprising a base 14, a facet 9 and a facet 10, these being plane and non-parallel. These facets 9 and 10 form between them an angle A, referred to as the angle of the prism. The facets 9 and 10 form, with the axis X of the optical guide 5, respectively, angles B and C. The facet 9 of a prism and the facet 10 of a consecutive prism together form a bottom angle D. The bottom angle D of each prism is in contact with a curve referred to as the bottom line 11. This bottom line 11 connects the vertex of all the angles D of the reflection face 6 of the optical guide 5. In other words, if it is considered that each prism 8 is a triangle, in its cross-section, the bottom line 11 connects the base 14 of each triangle with the base of the consecutive triangle.
It will be understood that the shape of each prism is considered as triangular in a 2-dimensional view.
FIG. 2 depicts, by means of arrows 12 and 13, an example of a path of a light ray propagating in an optical guide of the type of that described in the document U.S. Pat. No. 6,107,916. This light ray can be one of the light rays contained in the light beam 4 emitted by the light source 3. In this example, the light ray propagates in the optical guide 5 along a rectilinear initial path 12 until it encounters a facet of a prism. This path 12 forms, with the axis X of the optical guide 5, an angle of incidence E. When a light ray encounters a facet of a prism, for example the facet 10 of a prism 8 in the case of FIG. 2, the path 12 of the light ray is deviated by an angle F with respect to the initial path 12. The deviated path of the light ray is referenced 13. The deviation angle F between the path 12 and the path 13 is variable since it is related in particular to the angles of the prisms. Thus, the light ray is redirected, by the prisms 8 of the reflection face 7, towards the output face 6 of the optical guide 5.
In the example of FIG. 2, and for the purpose of simplifying this figure, the path of a single light ray has been depicted. It must be understood that other light rays with other paths can propagate in the optical guide, these rays possibly having been reflected one or more times, by one or more prisms or by the other face of the optical guide, before reaching a prism which redirects it towards the output face 6.
In the example of FIG. 2, the deviation 13 of the light ray corresponds to the principle of total reflection in an optical guide. The principle of total reflection is an optical phenomenon which allows the transmission of light in an optical guide 5. When a light ray passes from one medium to another medium having a different refractive index, its direction is changed; this is the effect of refraction. For a certain angle of incidence, and if the index of the initial medium is higher than that of the final medium, the light ray 12 is no longer refracted, it is totally reflected: total reflection is spoken of.
Thus, if FIG. 1 is considered again, it can be understood that the light beam 4 must be distributed over the entire length of the optical guide, that is to say between the end e1 and the end e2. However, some of the light beam 4 is lost, with constant prisms, since the flux which passes through the cross-section decreases as it propagates. It can therefore be understood that, at the end e2 of the optical guide 5, the amount of light lost is greater than at the end e1, close to the light source 3. In other words, the light throughput is lower at the end e2 than at the end e1 of the optical guide, the consequence of which is that a natural decrease occurs in the emitted light flux along the optical guide. Now, this decrease is visible to any person situated outside the vehicle.
Furthermore, in the example of FIG. 2, for an angle of incidence E between 0 and 5 degrees, the light ray undergoes total reflection. Thus, a light ray touching one of the facets 9 or 10 of the prism 8 is reflected towards the output face 6 of the optical guide 5, by the principle of total reflection.
In other words, light rays which arrive with an angle non-parallel to the axis X of the guide and, in particular, when they form an angle of 0° to 5° with this axis, are redirected towards the output face 6 of the optical guide by means of the prisms 8. The presence of the prisms 8 on the reflection face 7 of the optical guide 5 therefore makes it possible to make the light leave in the correct direction. By the principle of total reflection, the light ray is reflected towards the output face of the optical guide. In particular, it is reflected with a direction substantially perpendicular to the axis X of the optical guide 5, that is to say along the normal N to the axis X. Another direction of reflection of the light ray can be obtained by modifying the angle B and/or the angle C of the prism. In this case, if the angle between the ray leaving the optical guide and the normal N is referred to as G, then this angle G can only be positive. In other words, by modifying the slope of the prisms, it is possible to redirect the outgoing light rays so as to have a non-zero angle G.
However, in certain cases, it is advantageous to be able to send the light rays in a direction forming a negative angle G with the normal N. For example, in the case of FIG. 1, it can be seen that the optical guide 5 follows the profile of the protective glass 2 of the headlight 1. Consequently, the reflected light rays shine on the sides of the vehicle, in a direction Z. As can be understood in the view of FIG. 1, the light beam 4 emitted by the light source 3 propagates in the optical guide 5 to its opposite end e2. In proximity to this end e2, the light rays emitted perpendicular to the axis of the optical guide shine on the road laterally, and are seen by any observer on the side of the road. Thus, at this end e2 of the optical guide 5, the angle between the optical guide 5 and the desired direction of the light rays Y is not favourable. These light rays are lost, that is to say they are reflected towards a disadvantageous direction, which reduces the hoped-for performance of the lighting or signalling device.